Modeling of excitation dynamics in large-size molecular systems: Hierarchical equations with compartmentalization.

We describe the new method that can be useful for calculation of the excitation dynamics in large molecular arrays that can be split into compartments with weak exciton coupling between them. In this method, the dynamics within each compartment is evaluated nonperturbatively using hierarchical equations of motion (HEOM), whereas transfers between the exciton states belonging to different compartments are treated by the generalized Förster (gF) theory. In a combined HEOM-gF approach, the number of equations increases linearly when adding new compartments as opposed to pure HEOM, where a depth of hierarchy exhibits strong non-linear grows when scaling the total number of molecules.

Multiexciton spectra of molecular aggregates: application to photosynthetic antenna complexes.

We perform theoretical studies of nonlinear spectral responses of molecular aggregates upon multiple electronic excitations. It is shown that the transient absorption (TA) spectra exhibit gradual shifting to short wavelengths upon an increase in excitation energy accompanied by population of higher-order exciton manifolds. This transformation of the TA profile reflects a character of the exciton splitting and, therefore, is strongly dependent on the aggregate shape and size as well as on the exciton couplings and disorder of the site energies.

Excitation energy equilibration in a trimeric LHCII complex involves unusual pathways.

We explore the energy equilibration within the LHCII trimer using various approaches, including the Redfield-Förster method (with different compartmentalization schemes) and the exact hierarchical equation of motion (HEOM). We demonstrate that the inter-monomeric migration in the trimeric LHCII complex is not limited to direct transfers between quasi-equilibrated chlorophylls (Chls) , but also involves additional pathways with uphill transfers from Chls to the stromal-side Chls (connecting the Chls clusters from different monomeric subunits). Although these uphill transfers are slow they still can increase the total rate of inter-monomeric transfers by a factor of 1.

Resonant vibrations produce quantum bridge over high-energy states in heterogeneous antenna.

Photosynthetic light-harvesting complexes usually contain several pools of molecules with a big difference in transition energies, for example, chlorophylls a and b in plant antennas. Some pathways of the excitation energy transfer may include pigments from the low-energy pool separated by a site occupied by a high-energy molecule. We demonstrate that such pathways may be functional if high-frequency intramolecular vibrations fall in resonance with the energy gap between the neighboring molecules belonging to different pools.

Excitonic interactions and Stark fluorescence spectra.

We develop the theory for the Stark fluorescence (SF) of molecular aggregates by taking into account the mixing of the excited states [including the states with charge-transfer (CT) characters]. We use the sum-over-state approach and modified rotating wave approximation to describe interactions of the static and optical fields with the permanent and transition dipoles of the excited states. The SF spectral profiles are calculated using the standard and modified Redfield theories for the emission lineshapes.